def isPrime(a):     
    if a == 1: return False     
    test = True     
    k = a - 1     
    while k > 1:     
        if not a % k:     
            test = False                 
            break     
        k -= 1         
    return test    
       
n = input()   
r = []   
for i in range(len(n)):   
    for k in range(i + 1,len(n) + 1):   
        a = int(n[i:k])  
        if isPrime(a):   
            r += [a]   
print(max(r) if r else 0)
